1. Field of the Invention
The present invention relates to a wire electric discharge machining apparatus and a wire electric discharge machining method for effecting electric discharge between a wire electrode and a workpiece to machine the workpiece to have an arbitrary shape. The present invention also relates to a mold for extrusion produced by the wire electric discharge machining apparatus and the wire electric discharge machining method.
2. Description of the Related Art
In general, in wire electric discharge machining, which is one of the cutting machining procedures based on the use of a thin wire electrode, the discharge phenomenon is allowed to occur between the wire electrode and the workpiece. Therefore, wire electric discharge machining is advantageous in that the machining can be performed irrelevant to the characteristic values of a material such as the strength, the hardness, and the toughness of the workpiece. Further, wire electric discharge machining has the following advantages.
(1) The nonflammable working fluid other than oil can be effectively utilized.
(2) The electrode can be used in a disposable manner.
(3) The fatigue of the electrode does not greatly affect the machining accuracy.
(4) The CNC (computer numerical control) function can be sufficiently utilized.
Further, in wire electric discharge machining, a relatively small mechanical force is exerted on the workpiece during the machining. Therefore, wire electric discharge machining is also advantageous to machine thin workpieces, those having a fragile quality, and those having fine shapes such as extremely small holes and slits.
In order to improve the electric discharge efficiency in the travelling direction of the machining and improve the machining speed in the wire electric discharge machining described above, it is desirable to efficiently discharge undesirable matters such as sludge (machining powder), bubbles, and working fluid having lowered specific resistance generated in a minute gap as a machining gap between the wire electrode and the workpiece.
For this purpose, a method is available in which the fluid pressure of the working fluid to be jetted to the machining gap is increased. However, the increased fluid pressure makes a factor to give any vibration to the wire electrode. Therefore, this method involves drawbacks such as the decrease in machining accuracy, the generation of bubbles, and the fear of contact between the wire electrode and the workpiece to electrically form a short circuit resulting in electrode breakage.
A method has been hitherto suggested, in which the wire electric discharge machining is performed while efficiently discharging the sludge or the like by using a wire electrode having a specified shape. For example, Japanese Laid-Open Patent Publication No. 56-152531 describes the use of a wire provided with grooves, projections, and irregularities on its surface to discharge the sludge or the like. Japanese Laid-Open Utility Model Publication No. 62-110816 describes the use of a wire having a triangular, rectangular, or elliptic cross-sectional configuration so that the wire is rotated and fed to discharge the sludge or the like. Japanese Laid-Open Patent Publication No. 1-222821 describes the use of a wire having a straight angle cross section formed to give a helical configuration to discharge the sludge or the like.
In general, as shown in FIG. 13, wire electric discharge machining involves a standard machinable range which is the range .alpha. indicated by hatched lines. If the condition is deviated from this range, the following problems occur. That is, it is impossible to perform the machining to give a desired shape, the breaking of wire occurs, and the machining groove width is abnormally enlarged. For example, it is assumed that a machining groove having a width of 0.05 mm to 0.12 mm is intended to be formed on a workpiece having a thickness of 200 mm to 250 mm. This condition lies in a range .beta. indicated by dashed lines in FIG. 13, which is a condition far apart from the standard machining condition.
The machining, which is performed under a condition as described above, is exemplified by the formation of a mold for producing, by extrusion, honeycomb ceramics which is used, for example, for a catalyst for automobiles, especially, for example, the formation of a slit for defining the opening width of the cell of the honeycomb ceramics.
In such a case, the electric discharge machining can be performed with a prescribed machining groove at the early stage of the electric discharge machining. However, when the machining groove is deep, the sludge or the like tends to accumulate between the wire electrode and the machining groove. It is feared that any unstable electric discharge such as a secondary electric discharge may occur on account of the sludge to cause the enlargement of the machining groove width and the breaking of the wire electrode.
The purpose of the conventional method described above is that electric discharge machining is well performed persistently under the standard machining condition. The conventional method does not assume any electric discharge machining under conditions deviated from the standard machining condition as described above, because of the following reason. That is, the conventional method uses the wire having the specified shape from the early stage of the electric discharge machining (in which the standard machining condition is almost satisfied). Therefore, it is inevitable that the machining groove width is widened, which is not suitable to form the slit for defining the opening width of the cell of the honeycomb ceramics as described above.
Further, it is necessary to use a special power supply system directed to the wire electrode. Therefore, it is inevitable to change the arrangement of the wire electric discharge machining apparatus to a great extent. As a result, it is feared that the production cost may be increased.